1. Field of the Invention
This invention is in the field of atmospheric gas burners designed to operate over a wide range of inputs.
2. Discussion of the Background
It is often desirable in the design of gas-fired equipment, for instance instantaneous water heaters or circulating hot water boilers, to provide a gas fuel delivery apparatus that can automatically vary the flow of gas to the burner in response to a change in load. Regarding an instantaneous water heater, for example, water flows through the heat exchanger at variable rates depending on the hot water withdrawal rate at one or more remote taps. In addition to variable flow rate, the water may enter the heater at varying temperatures depending, for instance, on the season of the year. Since the intent of the heater is to deliver hot water at a specified temperature, it follows that the burner must deliver heat at a rate proportional to the flowrate through the heat exchanger and the temperature rise from inlet to outlet that accords with the desired outlet temperature. Many instantaneous water heaters incorporate a mechanism which varies the gas flowrate to the burner in response to changes in the load placed on the heater as described above.
A similar situation with regard to varying loads can pertain to hot water circulating boilers as well. In this case, the heat exchanger is part of a circuit through which water or some other fluid is pumped. In some instances, the flowrate through the heat exchanger can vary; for instance in a zone heating circuit served by one or more pumps. Also, a change in load can be reflected in a change in the temperature rise effected in the water passing through the heat exchanger. In some boiler applications, it is desirable to run the boiler at various outlet temperatures, depending for instance on the outdoor temperature (space heating application) or domestic hot water draw (for the case where the boiler also heats domestic water either directly or indirectly through another heat exchanger).
Such a heating appliance for domestic application requires a specially designed burner that can operate over a wide range of inputs. Cost considerations argue for an atmospheric Bunsen type burner as opposed to a power burner. A power burner can give higher turndown ratios, but is considerably more expensive to manufacture than an atmospheric burner. At present, the practical limit for turndown ratio for an atmospheric gas burner is about 5:1; that is to say, the minimum input at which the burner can acceptably operate is one-fifth of its maximum input.
For application to the aforementioned variable-input heating appliances, the operational criteria for an atmospheric burner are as follows:
1. It must perform acceptably over an input range of 5:1.
2. It should produce less than 200 ppm of carbon monoxide (on an airfree basis) over about 110% of its operating range. (This is somewhat more strict than the American Gas Association standard, but it is an achievable goal.)
3. It must light reliably with a spark ignitor or other ignition means and the flame must carry around to all ports after ignition, regardless of input.
4. The flame must be resistant to being blown out, and must carry back around after being blown off a portion of the ports, regardless of input.
5. The flame must not flash back into the mixer tube after the burner is shut off.
6. The flame must not lift or blow off the ports, and the burner should operate without excessive noise at all inputs.
7. The burner must not produce excessive yellow flame. This is a problem that can arise when using propane and other higher molecular weight gaseous fuels at high input. Even though the flame may be producing very little carbon monoxide under yellow flame conditions, the danger exists for soot deposition on any surface that is near a yellow flame.
It is desirable from the standpoint of manufacturing cost to fabricate the burner from sheet metal instead of castings or forgings. In addition to the savings in material, in using sheet metal, the ports can be punched rather than drilled, which considerably simplifies the production process.
However, making the burner head out of sheet metal introduces considerable difficulty insofar as meeting the aforementioned seven operational criteria, particularly at all inputs for a high-turndown burner. This is because of design tradeoffs; that is to say, moving the design in one direction in order to better meet one criterion can often have an adverse effect on another criterion. For example, in order to minimize blowoff and/or yellow flame problems at high input, the port area must be increased. However, if the port area is high, then the port velocity at low input will be very low, resulting in a marginal flame that is prone to being blown out and has poor flame carry-around characteristics. Another problem that sheet metal burners are particularly prone to is flashback. Flashback can occur when the gas-air velocity through the port is lower than the flame propagation velocity in the gas-air mixture. When this occurs, the flame has a tendency to propagate back through the port and into the burner head, resulting in a loud pop and flame shooting back out through the mixer tube. Flashback is most likely just after the burner is shut down from a low input. The port velocity starts to drop to zero, and the flame propagates back into the burner head, igniting the residual gas-air mixture inside. The reason sheet metal burners are particularly prone to this problem is that the port channel is very short. In a cast burner head with drilled ports, the port channel is long. When the flame starts to carry back through a long port channel, its temperature is quenched by the relatively cool metal of the burner head, and it is extinguished. With a short channel, the metal does not provide a sufficient heat sink to extinguish the flame before it gets through to the gas inside. The way the flashback problem is dealt with in sheet metal burners is to make the ports small. Small diameter circular ports can be used, and often narrow slotted ports are used.
A typical example of the sheet metal atmospheric gas burners designed for variable input, as applied to instantaneous water heaters, can be found in Hein, U.S. Pat. No. 3,512,910. This burner has multiple horizontal burner bars. A gas manifold has a discharge orifice for each burner bar. This burner meets the operational criteria, but is somewhat complex and expensive to manufacture. Another burner which uses similar horizontal burner bars, but has a single mixer tube and is fed from a single gas discharge orifice, is Norton et al., U.S. Pat. No. 4,723,907. Although it requires only one discharge orifice, this burner is still relatively complex and expensive to manufacture.